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Dying LiPo Battery produces Intermittent Voltage


the other day a friend of mine asked me if i could figure out why his bluetooth speaker was crackling and sometimes suddenly shutting itself down like there was no power.

I immediately checked the battery (LiPo, 3.7v) and indeed, it could not hold charge for a decent amount of time: when connected to a load ( in my case a 3.5v little incandescent bulb ), the voltage drops from 3.7 nominal to 2-ish in a minute or so. When disconnected, the open circuit voltage returns to 3.7 V.

The 2 volts were enough to still power the little bulb, until after some minutes, the bulb started turning on and off at a stable frequency, with a period of like a second, the voltage was switching between 2 and 0 volts (checked with oscilloscope).
I recorded a brief video of the phenomenon : https://imgur.com/a/yOqExz9
With time, the frequency began to slowly drop, as well as the voltage and so the light intensity, as someone would expect.

I'm really curious, I have just a basic knowledge of the working principle of a Lithium Ion Battery, but I can't figure out why this happens. Is this a normal behavior of dying LiPo batteries? And why it happens?

That lithium battery has a PCM in it.
It's not just a battery but a battery with a little circuit board containing an IC and a few transistors. It's intended to keep the battery safe.

Given that batteries age/condition your bulb might be too much of a load for it and the voltage drops below a safe level so the PCM disconnects your bulb.
Then the cell recovers its voltage (because there is no load any more) and the PCM reconnects your bulb when it thinks it's safe to do so.

Protected cells do this. Also, the phenomenon of "recovery" once disconnected is not unique to lion chemistry - I believe it has to do with the diffusion of the reactants. It sounds like that cell has far too much internal resistance to power its load.


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